Thumbs Up 

Thumbs are often credited as one of the great advantages in human evolution, allowing our ancestors to grip tools, manipulate objects, and reshape their environments.  

Now, scientists have found that squirrels, mice, and other rodents may have benefited from a similar advantage: Tiny thumbnails that helped them become one of the most successful groups of mammals on Earth. 

“Rodents make up almost half of the mammal species on Earth, and they’re found on every continent except Antarctica – their thumbnails might help explain why rodents became so successful,” said Anderson Feijó, co-author of a new study on the thumbnail, in a statement.

“Nuts are a very high-energy resource, but opening and eating them requires good manual dexterity that a lot of other animals don’t have – maybe rodents’ thumbnails allowed them to exploit this unique resource and then diversify broadly, because they were not competing with other animals for this food,” he added. 

For the study, a research team analyzed hundreds of specimens at Chicago’s Field Museum to map which rodents have thumbnails instead of claws.  

There are around 2,500 species of rodents worldwide, and these creatures make up 40 percent of all mammals. But until recently, scientists weren’t clear exactly how many of them had thumbnails. 

“Most people don’t (know). I didn’t,” quipped Rafaela Missagia, an author of the study. “I had studied rodents for years, and I didn’t know anything about their nails until I started working on this project at the Field Museum.”  

Missagia and her colleagues found that out of 433 genera examined, 86 percent of them had thumbnails. Their research examined both modern and prehistoric specimens going back as far as 55 million years ago. 

They found that, unlike sharp claws, the flat thumbnails gave rodents greater dexterity for gripping food and access to high-energy resources such as nuts and seeds. This ability would have fueled the group’s explosive diversification around the world. 

The findings also allowed researchers to build a family tree that suggests that all modern rodents descend from a common ancestor with thumbnails. 

Meanwhile, the team also discovered a link between lifestyle and the presence of thumbnails: Rodents with thumbnails tended to live above ground or in trees, while species with only claws were more often burrowers.  

That pattern echoes primates, which also evolved nails for life in trees – though the two lineages developed them independently in a striking case of convergent evolution. 

“This is a nice way to rethink what we consider a human characteristic,” co-author Anderson Feijó told the Washington Post. “We are showing with this paper that rodents actually have very good handling behavior, and we believe that their nail and thumb have played a very critical role in this.” 

For David Thybert of the University of East Anglia in the United Kingdom, the new study further highlighted the adaptability of rodents – some species can survive arid climates with little water, while others, such as the naked mole rat, are resistant to cancer. 

“They’re fascinating. … They have such fantastic traits that are sometimes similar to humans,” and are sometimes “almost extraterrestrial,” Thybert, who was not affiliated with the new research, told the Post.